Leading scientific thinkers of their time, such as Aristotle, Rene Descartes, Guillaume Duchenne, and Charles Darwin, have long promoted the idea that there are a handful of basic emotions that people express. In recent decades, that group has crystalized into six core emotions: happiness, surprise, sadness, anger, fear, and disgust.

But there are clearly many shades of gray between those emotions. For example, there’s the happy-because-I’m-eating-ice cream and the happy-because-I-just-learned-I-got-a-surprise-marriage-proposal looks, each of which is slightly different.

That’s what intrigued Aleix Martinez, associate professor of electrical and computer engineering at Ohio State University. “Six seemed a small number given the rainbow of possibilities of feeling and expressing emotions,” he says.